Transcript Sedimentary Textures

Weathering and the
Production of Sediment
Surface Portion of the
Geological Cycle
Types of Sedimentary Material
Types of Sedimentary Material
• Terrigenous Clastics (TC)
– Detrital Particles
– Transported by surface
processes to the site of
– Derived from pre-existing rocks
deposition
– Derived external to the
• Particulate Residues: quartz,
depositional basin
feldspar, rock fragments, etc
(unaltered rock forming
mineral/rock grains)
• Secondary Minerals: minerals
new-formed in the surface
weathering environment: clay
minerals, oxides,
amorphous silica, etc
Types of Sedimentary Material
• Allochemical Particles
formed in situ at the site of
deposition; of chemical/
biochemical origin
– Carbonates: ooids, fossil
fragments, pellets, lithoclasts
– Glauconite, phosphate :insitu
authigenic/particulate minerals
– Biogenic sediments: pelagic
tests, siliceous and calcareous
Types of Sedimentary Material
• Orthochemical Components
– Chemical Precipitates
• Secondary cement
• Primary chemical sediments: halite,
etc
• Organic Particulate Material
(detrital organic matter )
– terrestrial and particulate
– marine pelagic
– 95% found in mudrocks and indicative
of low Eh and low current strength
Laminated Castile Formation basinal
evaporites. Dark laminae are calcite
plus organic matter; light laminae
are gypsum (Peter Scholle)
Coal
Types of Sedimentary Material
• Pyroclasts
– particles fragmented and transported by volcanic
processes
• Tephra: tuff deposits
• Volcanic mudflows: lahar and volcanic breccia deposits
Tephra
Volcanic
Ash
Terrigenous Sediment
Sedimentary Analysis
• Goal:
– study modern analogue to understand processes
– identify processes which cause diagnostic
characteristic features
– unravel history
• Requires description (qualitative,
quantitative), analysis (graphical,
statistical), interpretation
Describing Siliciclastics
• Description
– Size
– Texture
– Fabric
• Analysis
• Maturity
– Textural
– Compositional
Describing Siliciclastics
(or how to have an intelligent discussion about rocks)
• Classification
• A necessary evil
• An attempt to organize wide variety into few classes
Useful
Expect deviations, overlap and some which just don’t fit
• Foundations
• Grain type
• Grain size
• Transported or in situ
• Different for each sediment type
Describing Siliciclastics-Size
• Size
• Gravel and larger (> 2 mm)
• Sand (1/16 - 2 mm)
• Mud (< 63mm = < 1/16 mm)
(conglomerate)
(sandstone)
(mudstone)
Conglomerate and Breccia
(> 2 mm)
Sand becomes sandstone
• (1/16 - 2 mm)
Mud becomes shale
• (< 63mm = < 1/16 mm)
Siliciclastic Rock
Classification: Texture
• Descriptive Textural Classification:
Ternary Plots
– G (gravel >2mm) - S (2mm>sand> 0.063mm)M (mud<0.063mm)
• significance of gravel (>30%) min. transport
energy
• S (sand) - C (clay<0.004mm)- S
(0.063mm>silt> 0.004mm
Siliciclastic Rock
Classification
• Mineralogical
Classification/terminology
– Sand ----------->Arenites
– CGL------------->Rudites
– MDST----------->Lutites
textural term mineralogical term
• Arenites Petrology
– Ease of analysis and sampling
– Composition can be interpreted
Describing Siliciclastics- Size
Wentworth scale
• Udden- Wentworth size scale
Udden, 1914; Wentworth, 1922
• Resolves problems with size classification
Cumbersome to discuss size
Limiting to restrict to 3 classes
• Four basic groups + modifiers make more
Clay (< 4 mm)
Silt (4 mm - 63 mm)
Sand (63mm - 2 mm)
Gravel (> 2 mm)
Siliciclastic Rock Classification:Texture
• Descriptive Textural Classification
– Grain Size
• Uden-Wentworth grain size scale
• Phi ()=-log2 (grain diameter in mm)
• naturally occurring groups;
Gravel ~ rock fragments,
Sand ~ individual mineral grains
(particulate residues)
Clay ~ chemical weathering products
(clay minerals, etc.)
Mud ~ particulate residues +/chemical weathering products
Describing Siliciclastics- Size
Wentworth scale (cont’)
– Subdivided scale by factor of 2
.0039 mm clay
.0078 mm very fine silt
128 mm = cobbles
256 mm = boulders
Logarithmic (base 2) progression!
 = -log2(grain diameter in mm)
As grain size increases, phi size decreases
Describing SiliciclasticsSedimentary Texture
• Aspects of texture
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Shape
Proportions of clastic: matrix
Degree of sorting
Surface texture
• Result of
– Parent rock type (shape)
– Weathering
– Transport history (sorting, shape)
•Generally for siliciclastics but can be useful for other types
Describing Siliciclastics
• Form/Shape
Zingg indices
spherical (equant), oblate (disk or tabular), bladed, prolate (roller)
• Roundness
• Degree of angularity
Function of transport history
Edges chip off as clasts knock into one another (progressive)
Estimate visually or calculate from cross- section
• Sphericity
•How closely clast approximates a sphere (equant)
Inherited feature! (function of shape formed in weathering)
slab may become discoidal but stays flat with time
Clastic Rock Classification
Texture: Sorting & Shape
• Sorting: measure of the
diversity of grain size
• A function of grain origin
and transport history
• Clast Rounding: surface
irregularity
– Due to prolonged agitation
during transport and reworking
Describing Siliciclastics
• Fabric
• Alignment of elongate clasts
• Anisotropic (preferred direction) arrangement of particles
• e.g., shale
• Surface Texture
• Pitted or not
• Folklore says eolian transport leads to etching
Yes!
No!
Describing Siliciclastics
Clastic: matrix
• Clasts
– Fragment which makes up a sedimentary rock
• Matrix
– Finer- grained material which lies between the
clasts
• Relative difference!
– Boulder/ cobble or sand/silt
Describing Siliciclastics
Degree of sorting
• Measure of distribution of clast sizes
• Well sorted
most clasts fall into one class on the Wentworth scale
• Poorly sorted
wide range of clast sizes
• Due to origin and transport history
• Greater distance (or repeated agitation of sediment),
better separation of sizes
• Qualitative (visual) and quantitative methods
Statistical/Graphic Presentation of
Texture: Grain Size/Sorting
• Quantitative assessment of the % of different grain
sizes in a clastic rock
– Mean: average particle size
– Mode: most abundant class size
Describing Siliciclastics
Grain size analysis
• Quantitative analysis
– (granulometric analysis)
• Quantitative assessment of % of different grain sizes in clastic
sediments and sedimentary rocks
– Useful in interpretting depositional history of clasts,
especially in modern environments
• Technique used varies with grain size
– Direct
– Indirect
Describing Siliciclastics
Grain size analysis- techniques
• Gravel
• direct measurement in the field
• measure all within a quadrant
•
Sand
meter is used for pebbles, cobbles
• pass through a stack of sieves with mesh keyed to 
weigh contents of each sieve, get distribution by wt.
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•
Sandstones and Conglomerates
(∑2d/n)/N
•n=#grains cut by view; d = diameter of field of view; N = total # views
counted
Coarse silt and finer
• based on Stokes Law
particle will settle through water at a predictable rate
• pipette
• sedigraph (X-ray the sediment/ water solution)
Describing Siliciclastics
Grain size analysis- graphic analysis
• Plots
– Histogram of weight percentage of size fractions
– Frequency curve
– Cumulative frequency curve
When plotted, grain size increases from right to left, fines to right, coarse to left
• Graphically represent grain size distribution
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mean grain size
standard deviation from a normal distribution (sorting)
symmetry (skewness)
flatness of curve (kurtosis)
Describing Siliciclastics
Grain size analysis- graphic analysis
• Different depositional environments exhibit
different grain size distributions
• Glacial sediments
poorly sorted
• River sediments
moderately sorted
• Beach sediments
well sorted
Statistical/Graphic Presentation of
Texture; Granulometry
Describing Siliciclastics
Grain size analysis- graphic analysis
• No unique solutions!
• Need additional data
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field observations
large- scale sedimentary relationships
sedimentary structures
facies associations
• If sediments are eroded from rocks previously
deposited, then misleading data can result
– e.g., river (mod. sorting) may be transported sediments
eroded from old beach rock (well sorted)
Significance of Grain Size, Sorting and Rounding :
Interpretive
• Textural Maturity
– Kinetic energy
during transport and
reworking
– Transport history
– Dispersal patterns
– Caveat emptor!
• Mixed sources
• Biogenic reworking
Describing Siliciclastics
Maturity of Siliciclastic Material
• Extent to which material has changed when
compared to the starting material (e.g., granite)
from which it was derived
• Textural
• Compositional/ mineralogical
– Generally linked
• High textural maturity leads to high compositional
maturity
Textural maturity
– Clue to
Erosion, Transport, Depositional history
– Independent of composition!
– Generalizations
•Maturity increases with energy input (same source)
higher downstream
•Relative to starting material!
clean sandstone can have high maturity (if rounded)
•Comparisons from different sources uncertain
different starting grain size and shape distributions not comparable
Compositional maturity
• Measure of proportion of resistant or stable minerals
present in the sediment, to less resistant minerals
• Sandstone with high maturity has mostly quartz
• Strongly influenced by composition of source rock area
Resistant
Quartz
Chert clasts
Zircon
Less resistant
Feldspar
Most other minerals
Lithic clasts
Cycles of Sedimentation
• First cycle
• Material is eroded, transported, deposited
• Additional cycles
•Burial, lithification, uplift, exposure, transport
•Redeposition - second cycle of sedimentation
•Increasing clastic detrital textural and mineralogical
maturity with each cycle
• Resistant minerals
•Can survive repeated weathering, erosion, transport
•Quartz, lithic fragments of chert, zircon (highly resistant)